The field of the invention relates to magnetic resonance imaging (“MRI”) systems. More particularly, the invention relates to an MRI system that shapes a fringe magnetic field in such a way that a low fringe magnetic region is provided which will not harm electronic equipment. It will be appreciated, however, that the invention is also amenable to other like applications.
MRI is a diagnostic imaging modality that does not rely on ionizing radiation. Instead, it uses strong (ideally) static magnetic fields, radio-frequency (RF) pulses of energy and magnetic field gradient waveforms. More specifically, MRI is a non-invasive procedure that uses nuclear magnetization and radio waves for producing internal pictures of a subject. Three-dimensional diagnostic image data is acquired for respective “slices” of an area of the subject under investigation. These slices of data typically provide structural detail having a resolution of one (1) millimeter or better.
When utilizing MRI to produce images, a technique is employed to obtain MRI signals from specific locations in the subject. Typically, the region that is to be imaged (region of interest) is scanned by a sequence of MRI measurement cycles, which vary according to the particular localization method being used. The resulting set of received MRI signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit MRI signals from specific locations in the subject. This is accomplished by employing gradient magnetic fields denoted GSLICE, GPHASE ENCODE, and GREADOUT which have the same direction as a polarizing field B0, but which are configured as needed to select the slice, phase encode and readout to facilitate the imaging. By controlling the strength of these gradients during each MRI cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting MRI signals can be identified.
An MRI system requires not only an intensive uniform magnetic field generator, typically less than 10 ppm within a 40–50 cm diameter spherical volume (“DSV”), but also a suite of associated electronics to operate the MRI system. Traditionally, almost all of the electronics are installed outside the imaging room and connected to the Magnet/Gradient/RF system through a penetration panel. The electronics are susceptible to damage and/or malfunctions due to the magnetic field, and are therefore usually placed outside the imaging room. There are drawbacks to this arrangement. The installation of an MRI system requires complex cable routing, often in excess of 150 cables, and a penetration panel construction, both of which lead to longer installation time and added expense. Also, because the electronics needed to operate the MRI system have been installed in a separate room, the set up and configuration of the electronics and the MRI system must be done onsite after or during installation, which requires additional time and the cost of personnel to perform the onsite set up and configuration.